In plastic pipe systems, connections between the various components must be sealed to prevent escape of fluids from the connections. The connections typically comprise an enlarged bell which receives a spigot. The spigot and bell can be formed as segments of pipe lengths or connecting members, such as elbows, T's, etc. In this application, the terms "fitting" and "pipe" are intended to include both pipe lengths and connecting members.
The seal between the bell and spigot is conventionally provided by a gasket mounted in one of the components of the connection and sealingly and resiliently engaging the other component. Most often, the seal is mounted in the bell and has at least one radially inwardly projecting sealing lip which is engaged and deformed by the spigot such that a tight interference fit is formed between the spigot and sealing lip to prevent the escape of fluid.
Gaskets formed exclusively of elastomeric material, such as natural rubber, are disadvantageous since they can be displaced from their intended position so as not to provide an adequate seal. For example, such gaskets may be pushed from their intended position during insertion of the spigot within the bell.
Gaskets reinforced with metal bands can be deformed and the metal can corrode causing the gaskets to leak. Moreover, when such gaskets are made an integral segment of the pipe spigot, deformation or corrosion of the metal band can cause the entire pipe section to leak.
Gaskets reinforced with plastic locking rings have been increasingly used. Such gaskets are described in U.S. Pat. No. 4,343,480 to Vassallo, which is hereby incorporated by reference in its entirety.
More recently, joining systems combining the use of a hard plastic forming segment, a retaining ring and an elastomeric gasket have been used. A system employing the combination of an EPSMI ring and a rubber gasket is disclosed in U.S. Pat. No. 4,723,905 to Vassallo and Valls, which is hereby incorporated by reference in its entirety. A further combination of a gasket having a hard plastic forming segment and an elastomeric gasket is shown in U.S. Pat. No. Des. 330,073 to Valls, which is hereby incorporated by reference in its entirety.
Although such combination forming segment/elastomeric gasket systems reduce the amount of soft rubber used, the cost of elastomeric materials, such as rubber, has significantly increased in recent years, thus forming a significant factor in the cost of fittings. While gaskets using locking rings and other systems including hard plastic contain less rubber than those made entirely of rubber, the rubber still forms a significant portion of the gasket and greatly affects its cost.
In addition to being expensive, soft rubber has a high compression set. Compression set causes rubber to lose some of its initial resistance or pressure in pipe joints and this effect reduces the capacity to seal especially under high pressure surges.